NRA: "Self-Defense: Understanding Ability, Opportunity and Intent"

First a disclaimer: I'm not your attorney and this is not legal advice. 

And now, from NRA Women: "Self-Defense: Understanding Ability, Opportunity and Intent."

• "You are not in sufficient danger to justify the use of deadly force unless the person attacking you has the actual physical ability to cause you bodily harm." 
• "You are not in sufficient danger to justify the use of deadly force unless the person attacking you has the immediate opportunity to cause you bodily harm." 
• "You are not in sufficient danger to justify the use of deadly force unless the person attacking you has demonstrated the intent to cause you bodily harm."

Be aware that Intent is usually a conscious decision, but not always, and that’s why some people prefer the word Jeopardy. Someone in the midst of a psychotic or drug-fueled episode might be unaware or not in control of what they’re doing, but your life could nonetheless be in danger by their actions, whether or not they really “want” to hurt you.   

Read the whole thing since it explains each concept in more detail and gives examples.  I would also note that, contrary to the wording used in the article, using deadly force is not just a matter of someone being able to cause "bodily harm." Self-defense requires proportionality of force--i.e., the force used to protect yourself or another must be proportionate to the harm faced or the force being used--so to use deadly force in self-defense requires more than a risk of "bodily harm," but what is often termed "death or grievous bodily harm" or "great bodily harm" or something similar.